On 31-Dec-06, at 6:33 AM, Rolf Østvik wrote:
Hi
I have a simple query which uses 32ms on 7.4.14 and 1015ms on 8.2.0.
I guess 7.4.14 creates a better execution plan than 8.2.0 for this
query but
i don't know how to get it to select a better one.
Explain analyse output will be found near the end of the e-mail.
(I have simplified my real query to get it as simple as possible.
The original query
contain 6 tables and was acceptable on 7.4.2, but took far too long
on 8.1.4)
I have made a test setup to compare 7.4.14, 8.1.4 and 8.2.0.
8.1.4 and 8.2.0 uses the same execution plan and same time to execute.
postgresql.conf values i changed is
7.4.14
Raised shared_buffers from 32MB to 128MB
Raised temp_buffers from 8MB to 32MB
8.2.0
Raised shared_buffers from 32MB to 128MB
Raised temp_buffers from 8MB to 32MB
Raised work_mem from 1MB to 8MB
set effective_cache to 3G
shared buffers should be 1G on this computer for 8.2
Dave
(It did however not have any influence of speed for
the view_subset query shown below.)
vacuum analyze has been executed.
Computer:
Dell PowerEdge 2950
openSUSE Linux 10.1
Intel(R) Xeon 3.00GHz
4GB memory
xfs filesystem on SAS disks
Table "public.step_result_subset"
Column | Type | Modifiers
-------------+---------+-----------
id | integer | not null
uut_result | integer |
step_parent | integer |
Indexes:
"step_result_subset_pkey" PRIMARY KEY, btree (id)
"step_result_subset_parent_key" btree (step_parent)
"step_result_uut_result_idx" btree (uut_result)
Table contain 17 179 506 rows, and is ~400M when exported to file
Table "public.uut_result_subset"
Column | Type | Modifiers
-----------------+-----------------------------+-----------
id | integer | not null
start_date_time | timestamp without time zone |
Indexes:
"uut_result_subset_pkey" PRIMARY KEY, btree (id)
"uut_result_subset_start_date_time_idx" btree (start_date_time)
Table contain ~176 555 rows, and is ~4.7M when exportd to file
Query is defined as view:
create view view_subset as
select
ur.id as ur_id,
sr.id as sr_id
from
uut_result_subset as ur
inner join step_result_subset as sr
on ur.id=sr.uut_result
where
ur.start_date_time > '2006-12-11'
and sr.step_parent=0;
Explain analyze is run several times to get a stable result
so i guess the numbers presented is with as much as possible
data in memory buffers.
Column step_result_subset.step_parent contain 0 in as many rows as
there are rows in table uut_result_subset.
(In my data set this will be 176 500 rows, Other values for
step_result_subset.step_parent is present 1003 times and lower.)
Query: "select * from view_subset;" run against 7.4.14 server.
QUERY PLAN
----------------------------------------------------------------------
--
Nested Loop (cost=0.00..1400.86 rows=17 width=8) (actual
time=0.161..26.287 rows=68 loops=1)
-> Index Scan using uut_result_subset_start_date_time_idx on
uut_result_subset ur (cost=0.00..63.28 rows=18 width=4) (actual
time=0.052..0.195 rows=68 loops=1)
Index Cond: (start_date_time > '2006-12-11
00:00:00'::timestamp without time zone)
-> Index Scan using step_result_uut_result_idx on
step_result_subset sr (cost=0.00..74.28 rows=2 width=8) (actual
time=0.149..0.379 rows=1 loops=68)
Index Cond: ("outer".id = sr.uut_result)
Filter: (step_parent = 0)
Total runtime: 26.379 ms
Query: "select * from view_subset;" run against 8.4.0 server.
QUERY PLAN
----------------------------------------------------------------------
Hash Join (cost=339.61..77103.61 rows=96 width=8) (actual
time=5.249..1010.669 rows=68 loops=1)
Hash Cond: (sr.uut_result = ur.id)
-> Index Scan using step_result_subset_parent_key on
step_result_subset sr (cost=0.00..76047.23 rows=143163 width=8)
(actual time=0.082..905.326 rows=176449 loops=1)
Index Cond: (step_parent = 0)
-> Hash (cost=339.31..339.31 rows=118 width=4) (actual
time=0.149..0.149 rows=68 loops=1)
-> Bitmap Heap Scan on uut_result_subset ur
(cost=4.90..339.31 rows=118 width=4) (actual time=0.060..0.099
rows=68 loops=1)
Recheck Cond: (start_date_time > '2006-12-11
00:00:00'::timestamp without time zone)
-> Bitmap Index Scan on
uut_result_subset_start_date_time_idx (cost=0.00..4.90 rows=118
width=0) (actual time=0.050..0.050 rows=68 loops=1)
Index Cond: (start_date_time > '2006-12-11
00:00:00'::timestamp without time zone)
Total runtime: 1010.775 ms
Thanks for tips.
Best regards
Rolf Østvik
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